Present-day methods for manufacture of MOS integrated circuits (ICs) do not yield MOS transistors having the desired uniformity of current/voltage characteristics over a plurality of ICs. Transistors designed to have the same characteristics display substantial non-uniformity of electrical characteristics over a plurality of ICs. For example, at a given voltage, the current in the saturation regime of two purportedly identical transistors may vary by a factor of three or more. However, the characteristics of identical transistors are substantially the same when both are found on a given integrated circuit. Usually, a distinction is made between transistors adapted to carry a stronger current than planned, and weak transistors adapted to carry a weaker current. The deviation of the actual current flowing through a transistor from its intended value, referred to as current drift, can be either positive or negative.
Current drift has undesirable consequences. Assume, for example, that an MOS transistor is commanded to charge or discharge an external bus or an internal line. The bus has an inductance L which conditions noise as a function of variations in the bus current Ib. The noise corresponds to the product L dIb/dt. The inductance L has a predetermined value. For an external bus, it includes the inductance of the parts of the bus in the transmitting and receiving housings, and the inductance of the link between the two housings. The value of the maximum permissible noise thus determines the maximum current intensity Ib. This intensity determines the size of the corresponding transistor. Assume that loading or unloading of the bus by this transistor triggers a threshold device such as a threshold inverter. The inverter will be triggered at the right moment by a typical transistor, at an earlier time by a relatively powerful transistor, and at a later time by a relatively weak transistor. However, a maximum duration is usually required for the operations in the integrated circuit to take place within times determined by a clock signal. If triggering takes place after the required maximum duration, operation will be disturbed and correction processes must be employed. Increasing the current of a weak transistor by increasing its size is impossible. In an integrated circuit with powerful transistors, this increase in size would lead to a current exceeding the maximum permissible value and produce excessive noise. Moreover, technological evolution requires increasingly rapid circuits. Operating speed would be improved if it were possible to effectively and reliably reduce the maximum time taken to command a functional element of the integrated circuit without increasing noise.